The Continuation monad represents computations in continuation-passing style
(CPS).
In continuation-passing style function result is not returned,
but instead is passed to another function,
received as a parameter (continuation).
Computations are built up from sequences
of nested continuations, terminated by a final continuation (often id)
which produces the final result.
Since continuations are functions which represent the future of a computation,
manipulation of the continuation functions can achieve complex manipulations
of the future of the computation,
such as interrupting a computation in the middle, aborting a portion
of a computation, restarting a computation, and interleaving execution of
computations.
The Continuation monad adapts CPS to the structure of a monad.

Before using the Continuation monad, be sure that you have
a firm understanding of continuation-passing style
and that continuations represent the best solution to your particular
design problem.
Many algorithms which require continuations in other languages do not require
them in Haskell, due to Haskell's lazy semantics.
Abuse of the Continuation monad can produce code that is impossible
to understand and maintain.

callCC (call-with-current-continuation)
calls a function with the current continuation as its argument.
Provides an escape continuation mechanism for use with Continuation monads.
Escape continuations allow to abort the current computation and return
a value immediately.
They achieve a similar effect to throwError
and catchError
within an Error monad.
Advantage of this function over calling return is that it makes
the continuation explicit,
allowing more flexibility and better control
(see examples in Control.Monad.Cont).

The standard idiom used with callCC is to provide a lambda-expression
to name the continuation. Then calling the named continuation anywhere
within its scope will escape from the computation,
even if it is many layers deep within nested computations.

Apply a function to transform the result of a continuation-passing
computation. This has a more restricted type than the map operations
for other monad transformers, because ContT does not define a functor
in the category of monads.

Action askString requests user to enter a string,
and passes it to the continuation.
askString takes as a parameter a continuation taking a string parameter,
and returning IO ().
Compare its signature to runContT definition.